Are you breaking your head to figure this out? To play the Sonomemories, you have to give them as food to your pet... I know it sounds weird, but it's the true. Don't worry, you will get them back when you stop playing them.
You can combine magic attacks. Two characters can attack one monster mixing two different kind of magic.
For example, make Forte use Water Splash and McBain Thunder Shine. The result will be a huge column of electrified water.
To select your pet, you have to get to the bottom of the well. When you get there, walk a little. Forte will stop and you will have two options to select.
"Was that a cat?"
"Was that a dog?"
Just select one option. Depending on your answer, you will have the selected pet in all your journey. Note that you can't change your pet later.
To import the chronology of the other two versions of the game, you need to find one of those "Tunning Forks". Interpret a song there and you will be teleported to a mysterious small room with three people there. talk to the one in the left side. He will ask you if you want to update your Chronology. Select "yes" and choose one of your game saves to get the data from.
As you noticed, the pet system is different from the other two games. Here you don't have to feed it using food... I know it sounds weird, but you feed it with weapons. The pet will have special effects in battle depending on what you gave it. For example, if you gave it a armor, the pet will raise your defense stats in battle.